[A_DAB_will_do]

I really appreciate you sharing your knowledge and experience. I was wondering if you can elaborate on some of your argon CO2 mix experience. I know that as the CO2 level approaches 5% you will get more carbon pickup. Did you ever play with a range between 2 to 5%? I know this is primarily spray but have you tried these gas blends with short-circuit? Also did you ever try running it on straight mild steel?

I know from prior experience on 409 stainless steel, that using anything more than 2.5% CO2 in the gas mix increases the possibility of Carbide Precipitation occurring. Higher CO2 mixes definitely shortened the lifespan of MIG welded exhaust headers and weldments around catalytic converter shells.

The performance with 3XX series stainless steels will differ; as the alloy chemistries of these grades are much higher than 409SS.

Speaking for myself, I don't care for the short circuit performance of 98/2 (CO2 or O2) gas mixes with stainless steel in typical open root pipe applications. Structural applications or more general purpose Fillet welds on stainless steel, or welds with backing bars, are OK with these mixes. But for open butt welds; the helium rich tri-mixes are much nicer to weld with and I think produce a better looking finished weld appearance. This is all relative to MIG welding; I still think TIG produces a much better [looking] result on stainless steel than MIG welding; if you can pay the penalty in lower productivity that comes with TIG welding.

 

[Weld_]

When I can get a second person to come and help I will try this. Would be fun for a local welding meet too if we ever get one setup. I have .045 in my machine. Don't have rollers for .030 on my Miller but have .023 (that might be interesting to try spray). I do have them for the ESAB though.

Whatever data you produce would be interesting. I don't know if you could get much success with .023 but it would be interesting to try.

 

[ManoKai]

@ Matt & Dave - excellent insight on real-world apps of short-circuit n spray on SS with blends of Ar, O2, CO2, and H.

 

[Weld_]

I know from prior experience on 409 stainless steel, that using anything more than 2.5% CO2 in the gas mix increases the possibility of Carbide Precipitation occurring. Higher CO2 mixes definitely shortened the lifespan of MIG welded exhaust headers and weldments around catalytic converter shells.

The performance with 3XX series stainless steels will differ; as the alloy chemistries of these grades are much higher than 409SS.

Speaking for myself, I don't care for the short circuit performance of 98/2 (CO2 or O2) gas mixes with stainless steel in typical open root pipe applications. Structural applications or more general purpose Fillet welds on stainless steel, or welds with backing bars, are OK with these mixes. But for open butt welds; the helium rich tri-mixes are much nicer to weld with and I think produce a better looking finished weld appearance. This is all relative to MIG welding; I still think TIG produces a much better [looking] result on stainless steel than MIG welding; if you can pay the penalty in lower productivity that comes with TIG welding.

I definitely think TIG is the way to go. I was wondering if you ever read or are familiar with the following information that is contained in this particular link. http://www.weldreality.com/MIG_welding_gases.htm

The claims that are made there are pretty interesting. Especially since the gentleman who is making the claims was one of the people responsible for AWS A5.32 MIG - GTAW Shielding Gas specifications (as he claims). Since you are a certified welding educator and certified welding inspector I would be interested in hearing your thoughts if you have the time to review the following link.

Thanks again for your time.

 

[Matt_Maguire]

I really appreciate you sharing your knowledge and experience. I was wondering if you can elaborate on some of your argon CO2 mix experience. I know that as the CO2 level approaches 5% you will get more carbon pickup. Did you ever play with a range between 2 to 5%? I know this is primarily spray but have you tried these gas blends with short-circuit? Also did you ever try running it on straight mild steel?

Back then you couldn't code much of anything with hard wire open arc mig (had a bad name for good reason).

Just used the Ar-5Co2 because it could short circuit & it pulsed pretty well (handy if you need to replace a sharp corner on something). Volt settings for short circuit are going to be LESS than what you'd use for straight up steel mig.

A DAB's right about trying to short circuit with Ar-2o2 or Co2 - not cool, that mix is for 'medieval type' spray using a #14 shade, leathers & putting some vasoline on the neck for burn protection. The stainless is worse than aluminum for smoking your tender self.

I get a kick out of Ed Craig, he's a shameless self promoter that loves the game (no sin I guess). Per the linked article he's pretty much right about the gas man sucking the money out of your account & into his. Better to look to europe for steady gas numbers - they don't seem to be dazzled by hype.

For instance (just checked some old rags)- The fabricating & welding handbooks that used to be published every 2 years for the AWS conventions had pretty much the latest trends & newest AWS spec changes & weld bling. In '84-5 the only short arc stainless mix listed in the gas section was the He tri-mix, and Ar-5o2 for spray, but there were no parameters listed for any hard wire mig in the stainless welding section. By '88-89 the stainless section shows He tri-mix for short arc & Ar-5o2 for spray with range parameters. In '90-91 shows the tri-mix & C25 for short arc and adds 1&2 o2 plus the 1&2 Co2 for spray with rifle shot parameters (full tables for each transfer type vs mix)... The Ar-5o2 had been dumped for spray BTW... There's a lot of bla-bla-bla that goes on out there & you're stuck sorting it out the hard way.

I remember the tin knocker "UA" shop down the street & their preference for electrode on heavy gage work rather than hard wire mig. They didn't get happy with mig-ing stainless until the cored wires got figured out & then they loved it. I'm pretty sure it was ease of use & appearance because their stick welds looked really nice - always.

Matt

 

[Weld_]

Back then you couldn't code much of anything with hard wire open arc mig (had a bad name for good reason).

Just used the Ar-5Co2 because it could short circuit & it pulsed pretty well (handy if you need to replace a sharp corner on something). Volt settings for short circuit are going to be LESS than what you'd use for straight up steel mig.

A DAB's right about trying to short circuit with Ar-2o2 or Co2 - not cool, that mix is for 'medieval type' spray using a #14 shade, leathers & putting some vasoline on the neck for burn protection. The stainless is worse than aluminum for smoking your tender self.

I get a kick out of Ed Craig, he's a shameless self promoter that loves the game (no sin I guess). Per the linked article he's pretty much right about the gas man sucking the money out of your account & into his. Better to look to europe for steady gas numbers - they don't seem to be dazzled by hype.

For instance (just checked some old rags)- The fabricating & welding handbooks that used to be published every 2 years for the AWS conventions had pretty much the latest trends & newest AWS spec changes & weld bling. In '84-5 the only short arc stainless mix listed in the gas section was the He tri-mix, and Ar-5o2 for spray, but there were no parameters listed for any hard wire mig in the stainless welding section. By '88-89 the stainless section shows He tri-mix for short arc & Ar-5o2 for spray with range parameters. In '90-91 shows the tri-mix & C25 for short arc and adds 1&2 o2 plus the 1&2 Co2 for spray with rifle shot parameters (full tables for each transfer type vs mix)... The Ar-5o2 had been dumped for spray BTW... There's a lot of bla-bla-bla that goes on out there & you're stuck sorting it out the hard way.

I remember the tin knocker "UA" shop down the street & their preference for electrode on heavy gage work rather than hard wire mig. They didn't get happy with mig-ing stainless until the cored wires got figured out & then they loved it. I'm pretty sure it was ease of use & appearance because their stick welds looked really nice - always.

Matt

Matt, I really appreciate your thoughts and opinions. Very insightful. As far as Ed Craig goes and his own promotion you can't blame a guy for trying to make a living. If you filter out a lot of the self-promotion nonsense out of his website it actually has a lot of really good knowledge and theory to explore. I am somewhat familiar with the European market as well and you are absolutely right they are not as easily sold on bells and whistles.

I am always looking further to educate myself and try new things. So in your opinion if you are going to MIG stainless (304,316) gauge metal (18-11) what would be your gas and process of choice (wire size, filler, etc.) and why.

Again thank you for your time and for sharing your experiences.

 

[A_DAB_will_do]

Matt, I really appreciate your thoughts and opinions. Very insightful. As far as Ed Craig goes and his own promotion you can't blame a guy for trying to make a living. If you filter out a lot of the self-promotion nonsense out of his website it actually has a lot of really good knowledge and theory to explore. I am somewhat familiar with the European market as well and you are absolutely right they are not as easily sold on bells and whistles.

I am always looking further to educate myself and try new things. So in your opinion if you are going to MIG stainless (304,316) gauge metal (18-11) what would be your gas and process of choice (wire size, filler, etc.) and why.

Again thank you for your time and for sharing your experiences.

I've enjoyed wading through Ed Craig's website on several occasions, and I too find myself agreeing with many things he has to say on the website. I haven't visited the site recently, but I the last time I found myself wishing that he'd hire someone to reorganize the entire thing. There's a great deal of good info there, but I find it tough to go back and find anything once I've left the site.

I know you asked Matt about choice of filler metal and process, but I'm going to throw in my two cents. The following factors would be important in making a choice:

Position of weld(flat, horizontal, vertical, overhead),
type of weld(butt, fillet, lap, etc) and
volume of weld(inches versus feet of weld deposit)

For versatility, I'm going to fall back on GTAW/TIG with 100% Argon gas.

If you're doing higher volume, repetitive welding then a smaller wire diameter(0.030" or 0.035"max) and some kind of spray transfer process. 98/2 Ar/CO2 would work if you can stay in the flat position by moving the weldment around. If you have to go out of position, then short circuit with Helium tri mix would be my first consideration. If you have the money and need the productivity then a pulsed spray transfer power supply is a more expensive option.

But again, type of weld joint, quality of fitup(optimal gap with little variation from joint to joint), and position of weld are all going to weigh on these choices.

Finished weld quality is also an issue. Do you have to concern yourself about burn through and sugaring on the back side of the weld? On 18 gauge material these are definite concerns that may need to be addressed with backing bars, back purge, or even some kind of post weld cleanup.

There are no universal solutions in my book; except TIG. But that versatility comes at a cost of time/productivity and the need for a skilled welder to get the full range of capability from the TIG process.

 

[Weld_]

I've enjoyed wading through Ed Craig's website on several occasions, and I too find myself agreeing with many things he has to say on the website. I haven't visited the site recently, but I the last time I found myself wishing that he'd hire someone to reorganize the entire thing. There's a great deal of good info there, but I find it tough to go back and find anything once I've left the site.

I know you asked Matt about choice of filler metal and process, but I'm going to throw in my two cents. The following factors would be important in making a choice:

Position of weld(flat, horizontal, vertical, overhead),
type of weld(butt, fillet, lap, etc) and
volume of weld(inches versus feet of weld deposit)

For versatility, I'm going to fall back on GTAW/TIG with 100% Argon gas.

If you're doing higher volume, repetitive welding then a smaller wire diameter(0.030" or 0.035"max) and some kind of spray transfer process. 98/2 Ar/CO2 would work if you can stay in the flat position by moving the weldment around. If you have to go out of position, then short circuit with Helium tri mix would be my first consideration. If you have the money and need the productivity then a pulsed spray transfer power supply is a more expensive option.

But again, type of weld joint, quality of fitup(optimal gap with little variation from joint to joint), and position of weld are all going to weigh on these choices.

Finished weld quality is also an issue. Do you have to concern yourself about burn through and sugaring on the back side of the weld? On 18 gauge material these are definite concerns that may need to be addressed with backing bars, back purge, or even some kind of post weld cleanup.

There are no universal solutions in my book; except TIG. But that versatility comes at a cost of time/productivity and the need for a skilled welder to get the full range of capability from the TIG process.

Your two cents are greatly welcomed and appreciated. I agree with you that Ed Craig's site is laid out terribly. But probably not any worse than ZORO or some of the other websites out there.

I agree with you on GTWA for anything that is low production and I am definitely fairly familiar with it and its versatility. What I am really trying to explore with this particular post is the gas options for MIG. Not only reading from ED's site but from practical hands-on experience I've always felt there were too many gas options, most of them at the very least being more expensive and less effective than the alternatives. I think most people unless they seriously get into the MIG process don't realize that there are literally dozens and dozens of gas mixes. To me that just never made any sense.


I find it appealing to be able to have an argon and CO2 bottle coupled with a mixer to tackle most of the MIG processes out there and just dialing in the correct mixture for that particular situation.

As always I want to thank you for sharing your experience and opinions and look forward to your contributions and anyone else who wishes to participate.

 

[Matt_Maguire]

Matt, I really appreciate your thoughts and opinions. Very insightful. As far as Ed Craig goes and his own promotion you can't blame a guy for trying to make a living. If you filter out a lot of the self-promotion nonsense out of his website it actually has a lot of really good knowledge and theory to explore. I am somewhat familiar with the European market as well and you are absolutely right they are not as easily sold on bells and whistles.

I am always looking further to educate myself and try new things. So in your opinion if you are going to MIG stainless (304,316) gauge metal (18-11) what would be your gas and process of choice (wire size, filler, etc.) and why.

Again thank you for your time and for sharing your experiences.

Saw a quote once that with Ed you needed to "eat the meat & then spit out the bones". :laugh::laugh::laugh:

Back in that day I wasn't the one spec-ing the process, we just had the large power supplies & could weld & manipulate large things & run automated. So the customer did the recipe - we had to try & make it work. Some was ASTM & the process was observed by an outside agent so it would qually. In the big picture we didn't do a lot of it (stainless) or cobalt chromes like stellite 6. That may explain some of my not being fond of them.

If it were me choosing today, no question I'd be looking for small cored wire for that 18-11 SS recipe. It's gonna have the better economy vs a tri-mix & hard wire. The only problem is it won't sit around forever, you need to use it up.

A long time ago I picked up a stainless training manual from praxair (actually I think the original was done by Linde). So I tried some google-foo & came up with this -> http://www.prest-o-sales.com/other_...es.com/other_links/gases/PDF/ShieldingGases/Manuals/GMAW of Stainless Steel.pdf

A little more foo and I found some gas-hat stuff here from the same -> http://www.prest-o-sales.com/other_...other_links/gases/PDF/ShieldingGases/Manuals/Shielding Gas Selection Manual.pdf

More foo yet & I tripped over GMAW steel -> http://www.prest-o-sales.com/other_links/gases/PDF/ShieldingGases/Manuals/GMAW of Carbon Steel.pdf

They don't give rifle shot settings for anything but do a pretty good job of giving the reader a general sense of what's going on & why. Maybe Manokai should add them to his sticky - I dunno.

Good luck
Matt

 

[Chris T.]

If those pieces got wasted that's definitely an expensive exercise, those are not cheap. I still think some sort of gas problem was the issue if they gave you a higher content CO2 that could have been your issue. Your LWS would not by any chance be Airgas, they are absolutely horrible at times.

The fitting was not wasted, just a lot of brushing to clean it up.

Yes, they were bought out by Airgas but still had a hard time with anything other than 75/25 or argon before that.

Sent from my SM-G920P using Tapatalk

 

[Weld_]

Saw a quote once that with Ed you needed to "eat the meat & then spit out the bones". :laugh::laugh::laugh:

Back in that day I wasn't the one spec-ing the process, we just had the large power supplies & could weld & manipulate large things & run automated. So the customer did the recipe - we had to try & make it work. Some was ASTM & the process was observed by an outside agent so it would qually. In the big picture we didn't do a lot of it (stainless) or cobalt chromes like stellite 6. That may explain some of my not being fond of them.

If it were me choosing today, no question I'd be looking for small cored wire for that 18-11 SS recipe. It's gonna have the better economy vs a tri-mix & hard wire. The only problem is it won't sit around forever, you need to use it up.

A long time ago I picked up a stainless training manual from praxair (actually I think the original was done by Linde). So I tried some google-foo & came up with this -> http://www.prest-o-sales.com/other_...es.com/other_links/gases/PDF/ShieldingGases/Manuals/GMAW of Stainless Steel.pdf

A little more foo and I found some gas-hat stuff here from the same -> http://www.prest-o-sales.com/other_...other_links/gases/PDF/ShieldingGases/Manuals/Shielding Gas Selection Manual.pdf

More foo yet & I tripped over GMAW steel -> http://www.prest-o-sales.com/other_links/gases/PDF/ShieldingGases/Manuals/GMAW of Carbon Steel.pdf

They don't give rifle shot settings for anything but do a pretty good job of giving the reader a general sense of what's going on & why. Maybe Manokai should add them to his sticky - I dunno.

Good luck
Matt

Matt,I really appreciate your input and those links. I'll read through them in the next few days. As far as core wire goes are you referring to something like UltraCore® FCP 308L, you have a particular recommendation?

 

[Weld_]

The fitting was not wasted, just a lot of brushing to clean it up.

Yes, they were bought out by Airgas but still had a hard time with anything other than 75/25 or argon before that.

Sent from my SM-G920P using Tapatalk

They are absolutely the worst at times and sometimes pretty decent. That is why the option of an argon cylinder and an CO2 cylinder coupled with the mixer is so appealing. It just simplifies a lot of stuff.

 

[Matt_Maguire]

@ weld - Had a chemical plant that liked “Core Bright” from alloy rods on their stuff – so whatever esab has that looks the same (I'd call them). The boys loved it – shiny, good bead shape and pretty much refused to “fingernail” at the toes.

I kinda wish I'd come across that praxair gas-hat manual a long time ago (I think). Beside the obvious thing of promoting 2&3 part mixes for mo' money, I'd have some questions – Like “why in the wide-wide world of sports” would you want a 2 part purge gas that has argon/hydrogen, or nitrogen/hydrogen blends (Pg 15&18) and HydroStar H-2 & 5 for back purging on stainless pipe (Pg 17&18) in the praxair guide??? WTF - I need some help here.

I think it's reasonable to worry about mig because there are more variables to keep track of to get it right, and it's real easy to go wrong.

I looked at some recommendations (WFS-Volts-Travel speed) & quickly crashed with the evolutions lincoln has over the last 25 years in their docs. From the GMAW manual GS-100 in '90 to the first C4.200 GMAW manual in 10/99 there was little change and they used tables from ASM ninth edition 1983. In the latest PDF version of C4.200 (on line) the tri-mix is the ONLY gas listed for short circuit. I'll post a scan of the old timey lincoln short arc tables for gage to 1/8” & general settings GMAW for SS.

So, being dazed and confused, I decided to look at my lincoln's synergic Ar(mix) settings and what IT thinks it should be doing and it's no where NEAR close to tri-mix settings in the docs... What's left is to wonder if they want Ar/o2, or Ar/Co2 & what balance. It's not in the manual, but for steel the tuning is C25 for short arc - and for pulse it likes C10.

I'm pretty sure - that it wants a more active gas than 98Ar/2Co2 but less than old timey C25 settings looking at the volts, and the jump when the WFS goes from 362IPM to 412IPM. If so then I s'pect the C25 that used to show up with 200-300 stainless was used when corrosion was a non factor, like architectural stuff?

Attached the settings for 035” wire using lincoln SS GMAW synergic mode (PM300) in the third column. The forth & fifth column are SS GMAW settings from the 1991-2 Weld & Fab handbook. In the second attachment notice the volt drops when using Ar/o2 -6V & Ar/Co2 -5V in the left panel.

Matt

 

[Silicon-based]

I compared the voltage settings for .035" stainless wire on the Power Wave machines, and the voltage for "Argon mix" is 3 - 4V less than the voltage for tri-mix in the short-arc range of wire feed speeds. Sounds pretty similar to your observations.

I don't personally like short-arc on stainless with Argon-CO2 mix that much. The usable wire feed range is limited, especially vertical-up, and the beads look somewhat cold even at the correct settings. I don't have much experience with tri-mix so I can't comment on that.

I usually use 97% Ar/3% CO2, what Airgas calls "Stainmix Gold".

John

 

[Weld_]

@ weld - Had a chemical plant that liked “Core Bright” from alloy rods on their stuff – so whatever esab has that looks the same (I'd call them). The boys loved it – shiny, good bead shape and pretty much refused to “fingernail” at the toes.

I kinda wish I'd come across that praxair gas-hat manual a long time ago (I think). Beside the obvious thing of promoting 2&3 part mixes for mo' money, I'd have some questions – Like “why in the wide-wide world of sports” would you want a 2 part purge gas that has argon/hydrogen, or nitrogen/hydrogen blends (Pg 15&18) and HydroStar H-2 & 5 for back purging on stainless pipe (Pg 17&18) in the praxair guide??? WTF - I need some help here.

I think it's reasonable to worry about mig because there are more variables to keep track of to get it right, and it's real easy to go wrong.

I looked at some recommendations (WFS-Volts-Travel speed) & quickly crashed with the evolutions lincoln has over the last 25 years in their docs. From the GMAW manual GS-100 in '90 to the first C4.200 GMAW manual in 10/99 there was little change and they used tables from ASM ninth edition 1983. In the latest PDF version of C4.200 (on line) the tri-mix is the ONLY gas listed for short circuit. I'll post a scan of the old timey lincoln short arc tables for gage to 1/8” & general settings GMAW for SS.

So, being dazed and confused, I decided to look at my lincoln's synergic Ar(mix) settings and what IT thinks it should be doing and it's no where NEAR close to tri-mix settings in the docs... What's left is to wonder if they want Ar/o2, or Ar/Co2 & what balance. It's not in the manual, but for steel the tuning is C25 for short arc - and for pulse it likes C10.

I'm pretty sure - that it wants a more active gas than 98Ar/2Co2 but less than old timey C25 settings looking at the volts, and the jump when the WFS goes from 362IPM to 412IPM. If so then I s'pect the C25 that used to show up with 200-300 stainless was used when corrosion was a non factor, like architectural stuff?

Attached the settings for 035” wire using lincoln SS GMAW synergic mode (PM300) in the third column. The forth & fifth column are SS GMAW settings from the 1991-2 Weld & Fab handbook. In the second attachment notice the volt drops when using Ar/o2 -6V & Ar/Co2 -5V in the left panel.

Matt

Matt thanks for sharing your input and experience. ESAB's wire is definitely top-notch. As a matter of fact so are there Stick electrodes if they were priced equally with Lincoln electric I would say they were better.

I never quite understood why you would need a two-part gas purge unless it is for agitation purposes on the rootside. Maybe someone in the community can chime in on this?

The more I read the more indication there is to being able to operate with just argon and CO2 along with a gas mixer. Again thank you for your contribution and I look forward to anything else you might have to add.

 

[A_DAB_will_do]

Matt thanks for sharing your input and experience. ESAB's wire is definitely top-notch. As a matter of fact so are there Stick electrodes if they were priced equally with Lincoln electric I would say they were better.

I never quite understood why you would need a two-part gas purge unless it is for agitation purposes on the rootside. Maybe someone in the community can chime in on this?

The more I read the more indication there is to being able to operate with just argon and CO2 along with a gas mixer. Again thank you for your contribution and I look forward to anything else you might have to add.

Weld_

Two part gas mixes for back-purge have some limited uses, in a few uncommon applications.

In the case of welding duplex stainless steels(and a few oddball austenitics), adding a little nitrogen to the back purge gas could affect how much ferrite is formed in the weld deposit. Nitrogen can be absorbed by the root bead, from the purge gas. Nitrogen increases the amount of austenite that is retained as the weld cools. More austenite microstructure in the weld deposit means less Ferrite. Too much Ferrite in a Duplex stainless steel weld can reduce the toughness and corrosion resistance of the weld deposit.

The effects of ferrite microstructure in austenitic stainless steels are more complicated than I can explain in detail. In brief ferrite levels can affect whether the weld deposit hot cracks or not... If you want to know more check out a book by OSU professor John Lippold on welding stainless steel.

In pipe welds, the root pass is the front line defense to corrosion caused by whatever is flowing through the pipe. So controlling the ferrite-austenite balance in the root pass can be critical for getting the best corrosion performance from the materials.

I've also read references to using hydrogen in the backpurge for austenitic and duplex stainless steel welds. Hydrogen is an extremely strong reducing agent; it grabs up any stray oxygen in the backpurge atmosphere, and prevents it from reacting with the weld deposit or surrounding HAZ. The bottom line is, it is a kind of insurance at getting the perfect bright silver root pass weld with imperfect purge conditions.

There's two potential problems with using hydrogen in the back purge.

One, if the weld deposit absorbs too much hydrogen and it doesn't have time to diffuse away as the weld cools, there is a chance of hydrogen assisted cracking in the weld or nearby base metal. This whole technique is controversial; the Europeans stainless folks swear by it and everyone else is deathly afraid of cracking problems. The other problem is that above about 4% by volume hydrogen gas is explosively flammable and not to be toyed with. I've read that some deal with this 'little problem' by only using gas mixes with 2% Hydrogen. They count on dilution to prevent fires. This works as long as the vented purge gas hydrogen doesn't rise up and accumulate in a confined space.

The second solution I reviewed was to use a 5% H2 gas purge and ignite the gas as it exits the purge vent; burning off the released hydrogen in a safe manner. This sounds crazy unless you've seen a hot tap on a natural gas pipeline. Then it doesn't look quite so insane.

I've used a 98%nitrogen/2%hydrogen gas mix once to back purge some duplex stainless steel. It was pretty much a wild experiment. But it does seem to work. It's not cheap, but I could see it helping if you couldn't get a solid back purge environment for some reason and if you wanted to move the ferrite numbers in the weld deposit and couldn't by any other means. I'm sure I wouldn't recommend it as a first choice for purging duplex stainless steel.

 

[Weld_]

Weld_

Two part gas mixes for back-purge have some limited uses, in a few uncommon applications.

In the case of welding duplex stainless steels(and a few oddball austenitics), adding a little nitrogen to the back purge gas could affect how much ferrite is formed in the weld deposit. Nitrogen can be absorbed by the root bead, from the purge gas. Nitrogen increases the amount of austenite that is retained as the weld cools. More austenite microstructure in the weld deposit means less Ferrite. Too much Ferrite in a Duplex stainless steel weld can reduce the toughness and corrosion resistance of the weld deposit.

The effects of ferrite microstructure in austenitic stainless steels are more complicated than I can explain in detail. In brief ferrite levels can affect whether the weld deposit hot cracks or not... If you want to know more check out a book by OSU professor John Lippold on welding stainless steel.

In pipe welds, the root pass is the front line defense to corrosion caused by whatever is flowing through the pipe. So controlling the ferrite-austenite balance in the root pass can be critical for getting the best corrosion performance from the materials.

I've also read references to using hydrogen in the backpurge for austenitic and duplex stainless steel welds. Hydrogen is an extremely strong reducing agent; it grabs up any stray oxygen in the backpurge atmosphere, and prevents it from reacting with the weld deposit or surrounding HAZ. The bottom line is, it is a kind of insurance at getting the perfect bright silver root pass weld with imperfect purge conditions.

There's two potential problems with using hydrogen in the back purge.

One, if the weld deposit absorbs too much hydrogen and it doesn't have time to diffuse away as the weld cools, there is a chance of hydrogen assisted cracking in the weld or nearby base metal. This whole technique is controversial; the Europeans stainless folks swear by it and everyone else is deathly afraid of cracking problems. The other problem is that above about 4% by volume hydrogen gas is explosively flammable and not to be toyed with. I've read that some deal with this 'little problem' by only using gas mixes with 2% Hydrogen. They count on dilution to prevent fires. This works as long as the vented purge gas hydrogen doesn't rise up and accumulate in a confined space.

The second solution I reviewed was to use a 5% H2 gas purge and ignite the gas as it exits the purge vent; burning off the released hydrogen in a safe manner. This sounds crazy unless you've seen a hot tap on a natural gas pipeline. Then it doesn't look quite so insane.

I've used a 98%nitrogen/2%hydrogen gas mix once to back purge some duplex stainless steel. It was pretty much a wild experiment. But it does seem to work. It's not cheap, but I could see it helping if you couldn't get a solid back purge environment for some reason and if you wanted to move the ferrite numbers in the weld deposit and couldn't by any other means. I'm sure I wouldn't recommend it as a first choice for purging duplex stainless steel.

I very much appreciate your contribution and sharing your experience and knowledge. I am familiar with hydrogen being burned from being around the petroleum industry a little bit. My fear with that would be is that you would have to use excessive flow rates to make sure the flame didn't go back into the purge environment, that would not be good.